feat(web): marketing theme toggle, site search palette, and phone dev access

- Theme toggle in the nav: follows the device until chosen, remembers the choice, sets data-theme before first paint (no flash).
- Site search: Cmd/Ctrl+K command palette on every marketing page, opened by the shortcut, a nav Search button, or the Docs search field. Searches pages, docs topics and articles that have a page.
- Docs topics move to content.ts so the page and the search index share one source.
- next.config: allowedDevOrigins for private network ranges so a phone on the same Wi-Fi hydrates the dev site. Dev only.
- Sun and moon icons added by hand (not Figma exports); noted in the icons README.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

web/next.config.ts

@@ -4,6 +4,9 @@ const apiOrigin = process.env.WURSOR_API_URL ?? 'http://localhost:3000';
 
 const config: NextConfig = {
   reactStrictMode: true,
+  // `next dev` blocks its dev endpoints for any host but localhost, so a phone on the same Wi-Fi gets the HTML but a page that
+  // never hydrates (menu, search and theme buttons all dead). Allow private-network addresses. Dev only; no effect on `next build`.
+  allowedDevOrigins: ['10.*.*.*', '192.168.*.*', '172.*.*.*'],
   // Do not let `next dev` write its own agent instructions into web/. The repository's AGENTS.md is the guide.
   agentRules: false,
   // Interim: the Sprint 1 shell at /app still calls the API on the same origin.